Dr. Alvarez noted that natural gas, which is increasingly available due to a boom in domestic shale gas production, has several potential environmental and economic advantages over coal and oil. However, he also emphasized that these benefits will only fully materialize if natural gas production and distribution is done correctly. He described this qualification as a “big if.”

Quantifying Greenhouse Gas Emissions Throughout the Supply Chain

Many people believe that natural gas can be a gateway fuel to catalyze the United States’ movement away from carbon-intensive fuels and toward a cleaner energy economy because natural gas-fired power plants emit only half as much carbon dioxide (CO2) per megawatt-hour of electricity as do coal fired ones.

Dr. Alvarez confirmed this potential benefit but stressed that CO2 emissions from electricity generation are only part of the story. In fact, natural gas production can also contribute to (worsen) climate change through greenhouse gas emissions that occur during earlier stages of the supply chain.

To fully understand the climate tradeoffs between different fuels, their associated greenhouse gas emissions must be accounted for at each stage of the fuels’ lifecycles, from production to use.

Accounting for Methane Leakage

Natural gas’ climate impacts are not limited to the CO2 emitted during combustion. Rather, natural gas also contributes to climate change if the gas (methane), itself a potent greenhouse gas, escapes or “leaks” to the atmosphere during earlier stages of the supply chain.

In comparing natural gas to coal or oil, it is essential to keep in mind the amounts and different warming properties of both CO2 and methane. Because CO2 and methane persist in the atmosphere on different timescales and contribute different levels of warming, the total emissions of both greenhouse gases from different fossil fuels and the properties of the gases themselves must be accounted for and compared.

Building a More Nuanced Accounting Model for Emissions

Prior studies have used Global Warming Potential (GWP) to compare the relative warming effects of different greenhouse gases and fossil fuels, but Dr. Alvarez argued that GWP can be misleading.

Specifically, because GWP looks at warming effects at only a single point in time (e.g., 100 or 200 years after emissions), it obscures the dynamics of emissions of different greenhouse gases, which, due to their specific lifecycles, create warming impacts on different timescales. For example, over a 100-year period, methane has 25 times the global warming impact of CO2 but if you consider a 20-year period instead, methane’s impacts are 72 times worse. These differences are due to the fact that an individual molecule of methane causes more warming than CO2 in the short-term, but methane also remains in the atmosphere for a comparatively short duration of only 12 years. CO2, on the other hand, causes less warming per molecule, but remains in the atmosphere much longer so its warming effects are more persistent.

To address this limitation, Dr. Alvarez and his colleagues have generated an alternative metric, which they dub Technology Warming Potential (TWP). TWP compares different fuels’ overall contributions to climate change across all greenhouse gases and timeframes. The figure below, taken from Dr. Alvarez’s study, shows the TWP for natural gas relative to three different fuel sources and uses.

The horizontal line on each graph (TWP = 1.0) represents the point at which natural gas has the same climate change impact as the alternative (conventional) fuel source. Points below this line signify that the climate impact of natural gas is less than that of the fuel to which it is being compared; points above signify higher climate impacts. For example, graph C shows that a natural gas power plant has roughly half the global warming impact of a coal power plant over a 200-year time horizon, but at time zero, natural gas’ climate benefits are lower—only 20 percent better than coal due to methane leakage.

Methane Leakage and Uncertainty in Leakage Rates

While the relative CO2 emissions of different fossil fuels are well known, a key unknown in the fuel comparisons is the rate of methane leakage from natural gas wells. Graphs A and B in the figure above assume that the rate of methane leakage from the natural gas supply chain is 3.0 percent of the total gas produced. Graph C assumes that this rate is 2.1 percent (the difference is based on varying assumptions about supply chains). The actual leakage rates are unknown, yet assumptions about natural gas’ benefits rely heavily on these leakage rates. For example, natural gas use in cars would provide climate benefits after 25 years instead of the 80 years shown in graph A if methane leakage were reduced from 3.0 to 2.0 percent. At 1.6 percent or less, natural gas would always have net climate benefits over gasoline.

The methane leakage rate is critical. As shown in the figure below (also from Dr. Alvarez’s study), when methane leakage rates, as represented on the y-axis, are lower, it takes less time to achieve net climate benefits from natural gas. The points at which the curves intersect the y-axis are the leakage thresholds below which natural gas has climate benefits over the conventional fuels for any time scale considered.

Dr. Alvarez emphasized that the threshold levels are not the only important points to consider. If, for example, the leakage rate of methane was found to be 4.0 percent, natural gas power plants would initially be worse for the climate than coal, but they would not be worse forever. In this scenario, beginning around 25 years after the conversion from coal-fired power plants to natural gas-fired ones—and continuing on into the future—there would be net climate benefits from having moved away from coal.

Summing Up

If natural gas is to replace other fossil fuels, then it is critical not only to understand the relative climate impacts of CO2 and methane, but also to find ways to minimize methane leakage in the natural gas supply chain. Dr. Alvarez concluded his presentation by mentioning that EDF is currently working with several companies in the natural gas industry to better measure and reduce the level of uncertainty about methane leakage at every stage of the natural gas production process. During the question and answer session, he mentioned that “green completions” of hydraulically-fractured shale gas wells could be a cost-effective strategy for reducing methane leakage. Green completions represent one key step in resolving the “big if” of whether natural gas production and distribution are being carried out correctly so that the theoretical climate benefits of natural gas can be realized in reality and when considered on any timeframe.

Dr. Alvarez’s powerpoint presentation is available for download here, and the webinar recording is available for viewing here: